The prevalence of hospital and community acquired Legionnaires' Disease in several well defined populations will be evaluated by determination of indirect immunofluorescent antibody titers in serial serum samples. Samples collected over several years from patients with hematologic malignancies and from chronic hemodialysis and transplant patients who are at risk for hospital acquired Legionnaires' Disease will be examined. The role of Legionnaires' Disease in children will be examined by antibody titer determinations in paired sera from cases of pediatric pneumonia of undetermined etiology. The role of soil and water exposure as risk factors for community acquired Legionnaires' Disease will be evaluated in two groups with intimate soil and water contact: highway maintenance and wastewater (sewage) workers, respectively. Sera collected before employment and at quarterly intervals after employment for up to three years will be examined. All of the samples in this study have already been collected for clinical use or for other investigations.